The invention relates to a method of optically detecting the roughness profile of a moved material surface, wherein a fine light bead is generated on the material surface and the reflected light is passed to a light receiving arrangement. The invention furthermore relates to an apparatus for carrying out the method.
Surface profiles are mainly determined by a direct mechanical profile measurement by means of the so-called stylus method using a diamond tip. This standard measuring method is admittedly universely usable for practically all technical surfaces, the method is however relatively slow and can easily lead to damage to the relevant material surface. It is furthermore unfavourable that a measurement cannot be effected on moving material.
Direct optical profile measuring methods have also already been used in which there is provided, for example, an optical scanning head with a fine light bead and follow-up focussing of the objective as a height measure (OPTIC COMMUNICATIONS, volume 31, No. 3, 1979), or optical scanning through a non-follow-up focus objective (EP No. 0 234 997) with the height dimension being obtained by the difference of two received signals. In the first case it is admittedly possible to obtain a contact-free universal optical measurement for all roughness ranges. The method is however in turn relatively slow (up to ca. 500 Hz) and relatively sensitive to adjustment and vibrations. In the second case the measurement can be carried out more rapidly. However here there is a restricted height measurement range and also a relatively high sensitivity to vibrations.
One can for example obtain an indirect pronouncement concerning the mean roughness through an optical detection of the scattering lobe generated by the material surface (DE No. 30 37 622 A1).
This simple integral method is practically insensitive to vibrations. It is however in particular a disadvantage that the measurement is restricted to profile heights which are smaller than the wavelength .lambda. of the light used and a pronouncement on the particular profile shape is not possible. Moreover, the measurement values that are obtained can be ambiguous and preclude, or make more difficult, a clear association to define roughness profiles or characteristic roughness numbers.